For the choux pastry
For the chocolate glacé icing
For the crème Chantilly filling
For the pastry, sift the flour and a pinch of salt into a large bowl and set aside.
Put 150ml/5½fl oz water and the butter into a saucepan set over a medium-high heat and stir until the butter melts. Allow the mixture to come to a rolling boil, then immediately remove the pan from the heat.
Add the flour and salt and beat very well with a wooden spoon until the mixture comes together.
Reduce the heat to medium and replace the saucepan over the heat. Stir for a minute until the mixture starts to slightly stick to the base of the pan. Remove from the heat and allow to cool for a minute.
Beat three eggs together and pour about one-quarter of the beaten egg into the pan. Using the wooden spoon, beat very well until incorporated. Add a little more egg and beat well again until the mixture comes back together. Continue to add the egg, beating vigorously all the time, until the mixture has softened, is nice and shiny and has a dropping consistency. You may not need to add all the egg or you may need a little extra. If the mixture is too stiff (not enough egg) then the choux pastries will be too heavy, but if the mixture is too wet (too much egg), they will not hold their shape when spooned onto baking paper.
Preheat the oven to 220C/425F/Gas 7. Line a baking tray with parchment paper (not greaseproof paper, as the éclairs may stick when cooked).
Place the dough into a piping bag fitted with the 5-8mm/¼ - 3/8in nozzle and pipe into lengths approximately 10cm/4in long onto the prepared baking tray, spaced about 4cm/1½in apart to allow for expansion. Use a small wet knife to stop the dough coming out when you have finished piping each éclair.
Beat the remaining egg and gently brush it over the éclairs. Bake in the oven for ten minutes, then reduce the oven temperature to 200C/400F/Gas 6 and continue to cook for a further 15–20 minutes, or until the éclairs are puffed up, golden-brown and crisp.
Remove the éclairs from the oven and, using a skewer or the tip of a small sharp knife, make a hole in the side or the base of each éclair. Return to the oven and bake for a further five minutes to allow the steam to escape. Transfer the éclairs to a wire rack to cool.
For the chocolate glacé icing, sift the icing sugar and cocoa powder into a bowl. Add 1-2 tablespoons of boiling water and stir to mix, adding a little more boiling water if necessary until the icing is spreadable but not too watery.
For the crème Chantilly filling, fold the sifted icing sugar and vanilla extract into the whipped cream. Chill in the fridge until you are ready to use it.
When the éclairs are cool, spoon the crème chantilly into a clean piping bag fitted with the small, plain nozzle and pipe the cream into the éclairs through the hole made during cooking until they are well filled.
Using a small palette knife or table knife that has been standing in a jug of hot water (to make spreading the icing easier), spread the icing over the top of each éclair, dipping the knife into the hot water between each éclair. Serve.